Scala: assert vs. require

// 1. let's compare assert, assume, require, ensuring
object Main extends App {
  private val days = List("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")

  def printAllDays() {
    assert(days.length == 7)           // usual assert (what we expect 100%), can be skipped in PROD
    days foreach println               // for a static analysing tool, assert is a predicate to be proven
  }

  def factorial(n: Long): Long = {
    assume(n >= 0)                     // the same as assert, can be skipped in PROD
    n match {                          // for a static analysing tool, assume is an axiom
      case 0 => 1
      case _ => n * factorial(n - 1)
    }
  }

  def getAverageAge(ages: Int*): Int = {
    require(ages forall {_ >= 0})      // require from a caller that all ages must be non-negative! Checked in PROD!
    ages match {
      case Nil => 0
      case _ => ages.sum / ages.length
    }
  } ensuring (result => result >= 0)   // ensure that everything is OK. Checked in PROD


  printAllDays()
  println(factorial(5))
  println(getAverageAge(1, 2, 3, 4, 5))
}

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// 120
// 3



// 2. Now let's violate days:
private val days = List("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "XYZ")

// Output:
// Exception in thread "main" java.lang.AssertionError: assertion failed
//  at scala.Predef$.assert(Predef.scala:204)
//  at ru.mitrakov.self.sandbox.Main$.printAllDays(Main.scala:8)



// 3. Now let's violate factorial:
assume(n >= 10)

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// Exception in thread "main" java.lang.AssertionError: assumption failed
//  at scala.Predef$.assume(Predef.scala:235)
//  at ru.mitrakov.self.sandbox.Main$.factorial(Main.scala:13)



// 4. Now let's pass incorrect parameters:
println(getAverageAge(1, 2, 3, 4, 5, -1))

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// 120
// Exception in thread "main" java.lang.IllegalArgumentException: requirement failed
//  at scala.Predef$.require(Predef.scala:264)
//  at ru.mitrakov.self.sandbox.Main$.getAverageAge(Main.scala:21)



// 5. Now let's change ensuring condition:
} ensuring (result => result >= 10)

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// 120
// Exception in thread "main" java.lang.AssertionError: assertion failed
//  at scala.Predef$.assert(Predef.scala:204)
//  at scala.Predef$Ensuring$.ensuring$extension2(Predef.scala:316)
//  at ru.mitrakov.self.sandbox.Main$.getAverageAge(Main.scala:26)



// 6. Now let's violate both assert and assume:
private val days = List("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "XYZ")
...
assume(n >= 10)
// Re-compile project with a compiler flag: -Xdisable-assertions
// E.g. in IntelliJ IDEA:
// Settings -> Build, Execution, Deployment -> Compiler -> Scala Compiler -> Additional compiler options

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// XYZ
// 120
// 3



// 7. But requirement will still be checked!
println(getAverageAge(1, 2, 3, 4, 5, -1))

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// XYZ
// 120
// Exception in thread "main" java.lang.IllegalArgumentException: requirement failed
//  at scala.Predef$.require(Predef.scala:264)
//  at ru.mitrakov.self.sandbox.Main$.getAverageAge(Main.scala:21)



// 8. And 'ensuring' too! It may be confusing, because 'ensuring' is implemented
// via 'assert', but it has NO '@elidable' annotation, so it will throw an exception!
} ensuring (result => result >= 10)

// Output:
// Sunday
// Monday
// Tuesday
// Wednesday
// Thursday
// Friday
// Saturday
// XYZ
// 120
// Exception in thread "main" java.lang.AssertionError: assertion failed
//  at scala.Predef$.assert(Predef.scala:204)
//  at scala.Predef$Ensuring$.ensuring$extension2(Predef.scala:316)
//  at ru.mitrakov.self.sandbox.Main$.getAverageAge(Main.scala:26)



// P.S. think of assert as InternalServerError (500), while of require - as BadRequest(400)